1. Field of the Invention
This invention relates generally to electrochemical machining and more particularly to shaped electrodes useful in an electrochemical process known as Shaped Tube Electrochemical Machining.
2. Description of the Related art
Gas turbine engines are used for a wide variety of aeronautical, marine and industrial applications. Generally, a gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and the mixture is ignited for generating hot combustion gases. These gases flow downstream to a turbine section that extracts energy therefrom to drive the compressor and provide useful work. It is well known to increase the efficiency of gas turbine engines by increasing the turbine operating temperature. As operating temperatures are increased, the thermal limits of certain engine components may be exceeded, resulting in material failure or, at the very least, reduced service life. In addition, the increased thermal expansion and contraction of these components adversely effects clearances and their interfitting relationships with other components having different thermal coefficients of expansion. Consequently, these components are cooled to avoid potentially damaging consequences at elevated operating temperatures.
It is common practice to extract a portion of the compressor discharge air for such cooling purposes. Turbine components disposed in the hot gas path typically employ internal cooling to keep the component temperatures within certain design limits. Internal cooling of turbine components usually involves a number of long, small diameter holes formed in the component walls. One common approach to forming these cooling holes is an electrochemical process known in the industry as the Shaped Tube Electrochemical Machining (STEM) drilling process.
The STEM drilling process is a variation of electrochemical machining (ECM) that involves providing a tube or hollow electrode through which an electrolyte, such as sulfuric or nitric acid, is pumped under a predetermined pressure onto a surface area of a conductive workpiece where a hole is to be drilled. A direct current electrical potential is established between the electrode and the workpiece to cause controlled deplating of the electrically conductive workpiece when the electrolyte flows from the electrode onto the workpiece. The deplating action takes place in an electrolytic cell formed by the negatively charged electrode (cathode) and the positively charged workpiece (anode) separated by the flowing electrolyte. Traditionally, STEM drilling of turbine components involved using round or circular electrodes to bore the cooling holes as straight cylindrical apertures.
In the continuing effort to improve the thermodynamic efficiency of gas turbine engines by further increasing gas inlet temperatures, the cooling efficiency of conventional circular holes is not always sufficient. This is particularly so in turbine rotor blades (sometimes referred to as buckets) where, given the constraints of the wall thickness at the blade trailing edge, it is extremely advantageous to use elliptic or oval shaped cooling holes to maximize the wall thickness while maintaining the required amount of coolant flow. Other non-circular geometries may also be useful. In such instances, tubular electrodes having the desired non-circular shape are substituted for conventional circular STEM electrodes. However, non-circular tube geometries typically develop higher internal stresses than circular tube geometries during manufacture thereof. These internal stresses tend to cause electrode bending, requiring the electrode to be straightened prior to a STEM drilling operation. It can be difficult to straighten full-length, non-circular electrodes prior to a STEM drilling operation and to keep such electrodes straight during the drilling process. Furthermore, non-circular electrodes may also experience pressure imbalances of the electrolyte flowing therethrough due to the non-axisymmetric nature of such electrodes. Such pressure imbalances can add to the difficulty of keeping electrodes straight during the drilling process. Non-circular STEM electrodes are also relatively expensive to manufacture.
Accordingly, it would be desirable to provide a STEM electrode that is capable of forming non-circular holes while avoiding the cost and straightening problems experienced with full-length, non-circular electrodes.
The above-mentioned need is met by the present invention, which provides an electrode for use in an electrochemical machining apparatus. The electrode includes a hollow shank having a circular cross-section and a hollow tip section having a non-circular cross-section disposed on one end of the shank. The tip section is very small in length relative to the shank such that the shank comprises a large majority of the electrode""s overall length.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.